missing plugins functionality - add network tests (network-verify)

[Michał Górniak]

Hey,
when we create plugins (specially SDN but not only) it will be amazing to have possibility to add some custom network test which will be run during network verify.

Example of usage:
when we have juniper contrail plugin we need to define GW ip but we didn't check if the ip is reachable etc and if its not its hard to change it later ;).